The present invention relates to hydrocarbon well logging systems and methods; more particularly, it relates to a computer-based system and method for acquisition, presentation, processing, and recording of nuclear hydrocarbon well logging data.
Well logging systems have been utilized in hydrocarbon exploration for many years. Such systems provide data for use by geologists and petroleum engineers in making many determinations pertinent to hydrocarbon exploration. In particular, the systems provide data for subsurface structural mapping, defining the lithology of subsurface formations, identifying hydrocarbon productive zones, and interpreting reservoir characteristics and contents. Many types of well logging systems exist which measure different formation parameters such as conductivity, travel time of acoustic waves within the formation, and the like.
Still another class of systems seeks to measure incidence of nuclear particles on the well logging tool from the formation for purposes well known in the art. These systems take various forms, including those measuring natural gamma rays from the formation. Still other systems measure gamma rays in the formation caused by bursts of neutrons into the formation by a neutron source carried by the tool and pulsed at a preselected interval.
In these nuclear well logging systems, reliance is made upon the physical phenomenon that the magnitude of energies given off by a nucleus resulting from natural radioactive decay or induced nuclear radiation is indicative of the presence of certain elements within the formation. In other words, formation elements will react in predictable ways, for example, when high energy neutrons on the order of 14.2 Mev collide with the elements' nuclei. Different elements in the formation may thus be identified from characteristic gamma ray energy levels released as a result of this neutron bombardment. Thus, the number of gamma rays at each energy level will be functionally related to the quantity of each element present in the formation such as the element carbon which is present in hydrocarbons. The presence of gamma rays at a 2.2 mev energy level may, for example, indicate the presence of hydrogen, whereas predominance of gamma rays having energy levels of 4.43 and 6.13 Mev, for example, may indicate the presence of carbon or oxygen.
Accordingly, in these nuclear well logging systems, it is frequently useful to not only detect occurrence of such particles, but to further obtain data regarding their spectral distributions both in terms of time and energy. Such data can yield extremely valuable information about the formation, such as identification of lithologies which are potentially hydrocarbon producing. Moreover, this desired spectral data may not be limited to that of natural gamma rays for example, but also may be desired for the gamma ray spectra caused by bombardment of the formation with the aforementioned pulse neutron sources.
Prior art well logging systems for conducting spectral analysis of nuclear particles have conventionally included a subsurface well logging instrument which traverses a well borehole. The instrument typically includes a gamma spectrometer including a thallium-activated sodium iodide crystal optically coupled to a photomultiplier tube. A high voltage supply accelerates deuterons into a tritium target, generating a large number of 14.2 Mev neutrons, this pulsed neutron source being activated at repetition rates of 20,000 bursts per second. Subsequent gamma radiation from the formation incident upon and detected by this high resolution scintillation crystal generates a pulse of light which in turn causes the photomultiplier tube to generate electrical pulses each proportional to the gamma ray energy causing the pulse. The scintillation spectrometer, comprised of the detector-photomultiplier tube, is maintained at a low temperature in thermal isolation in a Dewar-type flask.
As the photomultiplier tube generates these electrical signals, a downhole electronic amplifier provides voltage amplification and transmits the detector voltage pulse signals in analog form uphole on a single or multi-conductor logging cable to surface instrumentation for analysis and storage. At the surface, this pulsed information is amplified and routed to a multi-channel spectral analyzer system for deriving the desired spectra. The surface multi-channel analyzer provides a total pulse count and selects pulses within prescribed energy windows for separate counting. In addition, the multi-channel analyzer may also generate a time spectrum of the detected gamma ray pulses yielding pulse counts within prescribed time windows. In one variation on the aforementioned systems, rather than sending the actual analog voltage pulses to the surface from the downhole spectrometer, in some instances systems are provided wherein each pulse is first digitized downhole, and the digitized value of the height of each pulse is transmitted to the surface for analysis.
One particular form of well logging system for performing spectral analysis of induced gamma rays is the carbon-oxygen log wherein a ratio of detected elements is used to indicate presence or absence of hydrocarbons, among other parameters. In this system, the aforementioned pulsed neutron source produces high energy neutrons which are delivered into the formation. As previously noted, when carbon and oxygen are bombarded, as with other elements, by high energy neutrons, they emit gamma rays characteristic of their respective nuclei, with the carbon gamma ray energy being 4.43 Mev and the predominant oxygen gamma ray being 6.13 Mev. In like manner to the other systems, the gamma rays are detected downhole by a scintillation spectrometer calibrated to count pulses in the energy ranges most indicative of carbon and oxygen. Information available in the spectrum analyzed includes information of the inelastic gamma rays and calcium and silicon. Also, after the inelastic reactions have ceased, measurements of the gamma rays of capture of silicon and calcium are made.
Neutrons of sufficient energy to excite a carbon or oxygen nucleus are found to exist in a subsurface formation for only a brief period of time. Accordingly, the detector is gated and synchronized to make a measurement while neutrons are being emitted from the source. A carbon/oxygen ratio is derived by taking a ratio of the gamma ray counts in the selected energy windows.
The gamma ray measurements are presented in a conventional well log format comprising continuous plotter tracks. One track is used to monitor the output of the neutron source. Adjacent tracks contain a carbon/oxygen ratio curve, a silicon/calcium ratio curve, and an inelastic calcium/silicon ratio curve.
Well logging systems for measuring neutron absorption in a formation use a pulsed neutron source providing bursts of very fast, high-energy neutrons. Pulsing the neutron source permits the measurement of the macroscopic thermal neutron absoption capture cross-section (.SIGMA.) of a formation. The capture cross-section of a reservoir rock is indicative of the porosity, formation water salinity, and the quantity and type of hydrocarbons contained in the pore spaces.
Neutrons leaving the pulsed source interact with the surrounding materials and are slowed down. In a well logging environment, hydrogen in the surrounding water and hydrocarbons act to slow the neutrons. After the neutrons have been slowed to the thermal state, they are captured by atoms in the surrounding matter. Atoms capturing neutrons are in an excited state; and after a short time, gamma rays are emitted as the atom returns to a stable state.
The number of gamma rays present at any time is directly proportional to the number of thermal neutrons, i.e., the thermal neutron population. The decay rate of this neutron population is an exponential function, and is defined by specifying the time required for the thermal neutron population to decrease to one-half. This time is referred to as a neutron "half-lifetime". While it is actually the neutron lifetime that is measured, the more useful parameter is the capture cross-section. Capture cross-section and neutron lifetime are inversely related, with capture cross-section being a measure of the rate at which thermal neutrons are captured in the formation. Analysis of formation in this manner is referred to as "neutron decay analysis".
The measurement of neutron population decay rate is made cyclically. The neutron source is pulsed for 20-30 microseconds to create a neutron population. Since neutron population decay is a time-related function, only two time referenced gamma ray count measurements are necessary. The capture gamma rays are normally detected from time intervals that are 400-600 microseconds and 700-900 microseconds after each neutron burst. As the neutron source is pulsed and the measurements made, the subsurface well logging instrument is continuously pulled up the borehole.
The recorded log consists of four curves or tracks on a plotter. The capture gamma rays measured during the first measurement time period are recorded on one track. The capture gamma rays measured during the second measurement time period are recorded on a second track. On the third and fourth tracks, there are recorded a monitor of the neutron source output and the calculated capture cross-section. Capture cross-section is continuously calculated from the measurements made during the tWo measurement time periods.
Along with the thermal neutron log, an epithermal neutron log may be simultaneously recorded. Also, casing collars may be recorded.
Detailed discussion of such a carbon/oxygen digital well logging system as well as general theoretical background as to such logging operations may be found in U.S. Pat. No. 4,471,435 entitled "Computer-Based System for Acquisition of Nuclear Well Log Data" to Meisner, and Carbon/Oxygen Log, Publication No. 9417 of the Dresser Atlas Division of Dresser Industries, Inc., and comprising a collection of eight technical papers on the subject, both of which are incorporated herein by reference for all purposes.
The prior art nuclear well logging systems just described, though proving to be a very valuable tool in oil and gas exploration, have suffered from numerous deficiencies. First, with respect to the analog systems Which transmitted analog voltage pulses from the downhole spectrometer to the surface corresponding to each detected gamma ray, serious problems were encountered in pulse distortion and degradation due to limited band width on the conventional logging cables. Even with the previously described systems incorporating downhole digitization of each spectrometer pulse in an effort to avoid this pulse distortion, the system still transmitted the digital values for each pulse uphole, resulting in extremely slow system throughput. Due to the downhole instrumentation constraints of high temperature environments, low power availability, logging tool size constraints, and low signal-to-noise ratios, the approach of deriving downhole spectra was largely thought to be impractical if not impossible. Nevertheless, a well logging system and particularly a nuclear well logging system was highly desired which not only solved the pulse distortion and throughput problems, but provided better logging cable utilization which did not require the dedication of logging cable conductor time to sending the actual parameter values for each detected gamma ray pulse. It was further highly desirable to provide a nuclear well logging system with improved resolution, statistical accuracy, calibration and calibration maintenance characteristics. Still further, such a system was highly desirable which could, at the same time, provide for programmed downhole system flexibility as well as the opportunity for operator adjustment of parameters such as those affecting spectral generation including discriminator levels, detector gains, gate positions, source tracking, and temperature correction, as well as the potential for downhole spectral analysis, enhancement, and data manipulation under control from the surface or subsurface. It was still further desirable to simultaneously generate in addition to any one or a plurality of energy spectra such as the capture and inelastic spectra, various time distribution spectra for determination of capture cross-section, porosity, or the like. Moreover, it was further desirable to provide a system wherein the time spectrum of the neutron souce burst itself could be acquired. The present invention is directed to achieving these ends and in the promotion of consistent reproducible well logging spectral data at the surface.